As a bus for connecting an electronic device such as a personal computer to peripheral devices, there is known the USB (Universal Serial Bus) bus.
The bus, which is represented by the USB bus, is a serial transmission bus comprising a power line for supplying power to an electronic device connected thereto and a transmission line for transmitting/receiving data. The bus can employ unconstrained connection configuration such as the daisy chain and node branch, and can transmit data preferentially. Furthermore, employing the bus, cables can be attached or detached with an electronic device being supplied with power, and a network under the bus can be automatically restructured at the time when nodes are added or removed. Thus, the bus, which is represented by the USB bus, is used as an interface of an electronic device such as a VTR, a video camera, a color scanner, or a digital still camera which sends and receives multimedia data.
Respective electronic devices using the bus as an interface cannot freely consume arbitrary power, and are subject to power consumption stipulated based on specification given in advance. Especially, when there is no bus activity on the USB bus for more than a predetermined period of time due to suspension, etc. of an electronic device (referred to as stand-by state, hereinafter), power which can be consumed by the electronic device using the bus is restricted to 500 μA at most. So, in order to fulfill stipulated power consumption in the stand-by state, an electronic device is caused to transition into low power consumption mode in which power consumption of the electronic device is suppressed to the least.
As semiconductor processes are caused to be refined recently, even though an electronic device transitions into low power consumption mode, leak current increases in a semiconductor circuit of the electronic device, which cannot fulfill stipulated power consumption in the stand-by state. Especially, in a ASIC having a large-scale MCU (Memory Control Unit) or MPU (Micro Processing Unit) built therein, power consumption may increase to approximately 1 mA even in low power consumption mode.
On the other hand, in case power of an electronic device is shut down in the stand-by state, when resuming from the stand-by state, since a long time is required for rise time of a power source and for initialization time of a semiconductor circuit, there is raised a problem that the electronic device is judged to be disconnected from the bus by a connected external device.